Using principles in combination with our understanding of the climactic types, pedogenic regimes, and the soil-moisture regimes, we are now prepared to analyze the worldwide distribution of vegetation and to explain its variations with latitude, continental position and altitude.
The great biochores. All natural vegetation of the lands falls into four major structural subdivisions, the biochores. First is the forest biochore. We define a forest as a plant formation consisting of trees growing close together and forming a layer of foliage that largely shades the ground. Forests often show stratification, with more than one layer. Shading of the ground gives a distinctly different microclimate than that which would be found over open ground. No single value of precipitation can be stated because the effectiveness of the precipitation depends upon the water loss by evapotranspiration, and this in turn depends upon air temperature and humidity. Consequently, the forest biochore spans a great climatic range, from wet equatorial to cold sub arctic.
The savanna biochore is a formation consisting of a combination of trees and grassland in various proportions. The appearance of the vegetation can be described as parklike, with trees spaced singly or in small groups and surrounded by, or interspersed with, surfaces covered by grasses, or by some other plant life form, such as shrubs or annuals in a low layer. The savanna biochore indicates a climate of limited total annual precipitation with an uneven distribution throughout the year.
The grassland biochore consists of an upland vegetation largely or entirely of herbs, which may include grasses, grasslike plants, and forbs (broadleaf herbs). Degree of coverage may range from continuous to discontinuous and there may be stratification. The grassland biochore may include trees in moister habitats of valley floors and along stream courses where ground water is available. The grassland biochore is typical of a climate, which has small total annual precipitation, but otherwise the climate may range from one of extreme heat to one of extreme cold.
The desert biochore associated with climates of extreme aridity, has thinly dispersed plants and hence a high percentage of bare ground exposed to direct insolation and to the forces of wind and water erosion or to freeze thaw action. Although essentially treeless, the desert biochore may have scattered woody plants. Typically, however, the plants are small, e.g. herbs, bryoids, lichens. Because the desert biochore includes climate ranging from extremely hot tropical desert to extremely cold arctic desert, a great range in plant communities and habitats is spanned by the biochore.
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